Write equations to represent the following observations.

(a) A mixture of Mg(OH)₂(s) and Zn(OH)₂(s) is treated with NH₃(aq). The Zn(OH)₂ dissolves, but the Mg(OH)₂(s) is left behind.

(b) When NaOH(aq) is added to CuSO₄(aq), a pale blue precipitate forms. If NH₃(aq) is added, the precipitate redissolves, producing a solution with an intense deep blue color. If this deep blue solution is made acidic with HNO₃(aq), the color is converted back to pale blue.